GaN, AlN and InN which are nitride semiconductors, and materials including mix crystals thereof have wide band gaps, and may be used in high power electronic devices, short wavelength light-emitting devices or such. Thereamong, as a high power electronic device, a technology of a field effect transistor (FET), in particular, a high electron mobility transistor (HEMT), has been developed. Such a HEMT using a nitride semiconductor may be used in a high power and high efficiency amplifier, a high power switching device or such.
Since it is preferable that a HEMT used for such a purpose has a high drain withstand voltage and/or gate withstand voltage, a metal insulator semiconductor (MIS) structure, in which an insulating film to be used as a gate insulating film is formed, is used in many cases. By using such a MIS structure, it is possible to provide a semiconductor device suitable as a power semiconductor device.
Further, in a semiconductor device of such a field effect transistor, a protective film made of an insulator may usually be formed on the entire area of the surface after a gate electrode, a drain electrode or such is formed for the purpose of passivation or such.